When you want to use wood that is not naturally decay resistant in a wet application (outdoors, for example) or where it may be at risk for insect attack, you need to specify preservative-treated wood. This is lumber that has been chemically treated to make it unattractive to fungi and other pests. In the same way that you would specify galvanized steel where it would be at risk of rusting, you specify treated wood where it will be used in a setting conducive to decay. 

Wood does not deteriorate just because it gets wet. When wood breaks down, it is because an organism is eating it as food. Preservatives work by making the food source inedible to these organisms.

Properly preservative-treated wood can have 5 to 10 times the service life of untreated wood. This extension of life saves the equivalent of 12.5% of Canada’s annual log harvest.

Preserved wood is used most often for railroad ties, utility poles, marine piles, decks, fences and other outdoor applications. Various treatment methods and types of chemicals are available, depending on the attributes required in the particular application and the level of protection needed.

Links on Durability of Wood Products

Content

On-Line Information on Wood Design and Durability Information On Treated Wood

Information On Termites and Other Pests Information on Mould / Mold

Other Web Sites of Interest Books On Wood Design

ON-LINE INFORMATION ON WOOD DESIGN AND DURABILITY

Canadian Wood Council

Ottawa, Ontario, Canada

The Canadian Wood Council (CWC) is the national association representing Canadian manufacturers of wood products used in construction. CWC participates in building codes and standards committees relating to structural performance, fire safety and durability to ensure proper use of wood products. Designers, builders and building officials address their questions on the design and use of wood products and building systems to CWC’s technical staff, and CWC publishes a large array of manuals, brochures and electronic tools to provide guidance and resource material.

 

Web site: https://cwc.ca/

Helpdesk: (800) 463-5091

General: (613) 747-5544

Offers: Design handbooks, software, seminars, fact sheets, links and help. Almost everything is available on-line; some books need to be ordered. Information generally aimed at architects and engineers.

 

Canada Mortgage and Housing Corporation

Ottawa, Ontario, Canada

As Canada’s national housing agency, provides a wide range of services including leading-edge research. CMHC is Canada’s largest publisher of housing information and has Canada’s most comprehensive selection of information about homes and housing.

 

Web site: http://www.cmhc-schl.gc.ca/en/index.cfm

General calls, in Canada: (800) 668-2642

General calls, from outside Canada: (613) 748-2003

Offers: Best practice guidelines and other publications. Information aimed at the general public as well as designers and builders. Publications must be ordered, either by phone or on-line. On the web site, go to “order desk” to browse catalog and/or purchase on-line. See sub-sections on Design and Construction, Renovation, Healthy Housing and Multi-unit Design for titles of interest. Recommended: “Best Practice Guide, Wood Frame Envelopes in the Coastal Climate of British Columbia” which will be found at Order Desk/Multi-unit Design. CMHC also publishes a two-volume set “Building Envelope Rehabilitation: Consultant Guide and Owner/Property Manager Guide” which provides help in assessing and fixing moisture-related damage in occupied buildings. CMHC’s web site additionally provides quite a bit of information on-line, in brief web pages – from the home page, go to “browse by topic” and click on “building, renovating and maintaining.”

 

University of Massachusetts, Building Materials and Wood Technology

Amherst, Massachusetts, USA

This web site for a university program primarily contains curriculum information, however click “publications” for plain-language information on many construction topics of general interest.

Web site: http://www.umass.edu/bmatwt/

Offers: Dozens of on-line articles aimed at the general public as well as designers and builders.

 

US Forest Products Laboratory

Madison, Wisconsin, USA

Established in 1910 by the U.S. Department of Agriculture Forest Service, the Forest Products Laboratory (FPL) serves the public as the U.S.’s leading wood research institute. FPL is recognized both nationally and internationally as an unbiased technical authority on wood science and use. Today, more than 250 scientists and support staff conduct research on expanded and diverse aspects of wood use. Research concentrates on pulp and paper products, housing and structural uses of wood, wood preservation, wood and fungi identification, and finishing and restoration of wood products.

Web site: http://www.fpl.fs.fed.us/

Phone: 608-231-9200

Offers: In addition to information about the research programs and technical research reports, the FPL web site also offers many on-line documents targeted for the general public. From the home page, click “FAQs” for access to the on-line (but somewhat technical) “Wood Handbook.” Click “Techlines” for a series of fact sheets that range from consumer-friendly to fairly technical. FPL is known as a source of information on wood finishes – click “Painting and Finishing Fact Sheets.” Almost all other FPL documents are also available on line. To find reports, papers or other documents, click “Search” (under Publications) on the home page and enter the topic of interest.

 

American Wood Council (AWC)

Washington, DC, USA

AWC is the wood products division of the American Forest & Paper Association (AF&PA). AWC’s mission is to increase the use of wood by assuring the broad regulatory acceptance of wood products, developing design tools and guidelines for wood construction, and influencing the development of public policies affecting the use of wood products.

Web site: http://www.awc.org/

Helpdesk: (202) 463-4713

General: (202) 463-2766

Offers: Has a very useful helpdesk for any questions regarding the use of wood and its products in building construction. Many publications are available online. Offer technical information regarding U.S. Codes and standards as well as a section on “Mold and Moisture in Homes.”

 

APA — The Engineered Wood Association

Tacoma, Washington, USA

A membership organization representing 75 percent of the structural wood panel products manufactured in North America, plus a host of engineered products that include glued laminated timber (glulam), composite panels, wood I-joists, and laminated veneer lumber. Very active in research and technology transfer.

Web site: http://www.apawood.org/

Tel: (253) 565-6600

Email: help@apawood.org

 

Offers: Hundreds of reports, fact sheets and other publications, generally geared for design and construction professionals. Many are available on-line; others can be purchased for a modest fee – search by topic area under “Publications.” Alternatively, browse the site by topic area for a wide range of information, on both specific engineered wood products (click “Products”) as well as general wood framing (click “Applications”). These topic-area pages are nicely cross-referenced to related publications. For specific durability-related information, click “Build a better home” in the left side menu. The web site also has FAQs and a “help desk.” Send questions by e-mail, or phone and ask for the help desk.

 

National Association of Home Builders Research Center

Upper Marlboro, Maryland, USA

Founded in 1964, the NAHB Research Center is a separately incorporated, wholly-owned, not-for-profit subsidiary of the U.S. National Association of Home Builders (NAHB), a membership organization of builders, manufacturers, and other housing industry professionals. Research spans a broad spectrum including design and construction of homes, land use, the environment, affordable and sustainable housing, and special needs housing. Technology transfer to builders and others is largely via “Toolbase,” a web site supplying information on building products, materials, new technologies, business management, and housing systems.

 

Web site: http://www.nahbrc.org/

Tel: (301) 249-4000

Toll Free in the US: (800) 638-8556

Toolbase help hotline: (800) 898-2842 (US only)

 

Offers: The Research Center web site offers 100 publication titles available for purchase, not available on- line. However, click on “Toolbase” and jump to a content-heavy web site with fact sheets, news and more all available on-line. Also has an “ask the expert” feature, by phone or e-mail.

 

Homeowner Protection Office

Vancouver, British Columbia, Canada

The Homeowner Protection Office (HPO) is a British Columbia Crown corporation formed in 1998 as a response to concerns about the quality of condominium construction in BC. Its purpose is to help strengthen consumer protection for buyers of new homes and improve the quality of residential construction in the province. The HPO is responsible for residential builder licensing, regulating mandatory third-party home warranty insurance, administering a no-interest repair loan program and tax relief grant for owners of leaky homes, and a research and education function designed to benefit the residential construction industry and consumers

Web site: http://www.hpo.bc.ca/

Tel: (800) 407-7757 (BC only)

Offers: This web site is full of on-line information specific to builder and consumer issues in BC. However, click on FAQs or Publications for access to several on-line documents of general interest to anyone involved in moisture-related damage repair.

 

National Research Council / Institute for Research in Construction (IRC)

Ottawa, Ontario, Canada

Established in 1947, IRC provides research, building code development, and materials evaluation services within Canada’s national laboratory. IRC’s Building Envelope and Structure program develops and applies technologies for design, construction and operation of durable, energy-efficient, and cost-effective building systems, in both new construction and repair or renovation, for all types of buildings. An interesting recent project is the Consortium for Moisture Management for Exterior Walls (MEWS), a research effort addressing some of the technical fundamentals behind moisture performance of building envelopes and funded by a partnership of industry groups. Results, when available, can be viewed on the web site.

 

Web site: http://www.nrc.ca/irc/bes/index.html

 

Offers: Click “publications” for access to a rich collection of on-line information, including nearly all of the famous (and not necessarily outdated) Canadian Building Digest articles published between 1960 and 1990. Also available on-line is IRC’s quarterly newsletter, staff articles from construction journals, and more. Some publications are abstracted only and can be ordered.

 

INFORMATION ON TREATED WOOD

American Wood Preservers’ Association

The American Wood-Preservers’ Association (AWPA) is an international, non-profit technical organization founded in 1904 to provide a common forum for the exchange of information for all segments of the wood preservation industry. AWPA provides standards development and a link for technical exchange between industry, researchers, and users of treated wood. As the principal standards writing organization for the wood treating industry in the United States, AWPA has significant influence internationally as well. AWPA Standards are written to insure that treated wood products perform satisfactorily for their intended uses. The Standards are recognized and used by customers worldwide, who purchase and use treated wood for applications in the building products, electrical, marine, railroad transportation, and road construction industries. The Standards, as well as technical papers contained in proceedings from annual meetings, are available for order through the web site, but may not be viewed on-line. However, there are several fact sheets and FAQs available on-line. Most of AWPA’s information is targeted to the wood treatment industry, but some material may be of interest to treated wood users.

 

Web site: http://www.awpa.com/

 

The Wood Preservation Science Council

Cambridge, Massachusetts, USA

Web site provides a thorough collection of on-line research documents (prepared by a variety of agencies) on the subject of health and environmental impacts of CCA-treated wood.

 

Web site: http://www.woodpreservativescience.org/facts.shtml

 

Wood Preservation Canada

Ottawa, Ontario, Canada

Wood Preservation Canada is a non-profit industry association comprising members from across Canada. Operating under a Federal Charter, the Institute serves as a forum for those involved with the wood preservation industry, from research to production, marketing and protection of the environment. WPC members cooperate with government departments and other agencies in preparing standards for the industry, and in developing guidelines for the design and operation of wood preservation facilities. It works with Canadian university testing laboratories, faculties and independent research organizations concerned with the development of treated wood. The web site offers many on-line fact sheets and FAQs targeted at users of treated wood. Also available on-line is a table of CSA O80 standards by commodity.

 

Web site: http://www.woodpreservation.ca/

 

International Research Group on Wood Preservation

The International Research Group on Wood Preservation (IRG) was launched as an independent research group in January 1969, with the Secretariat currently located in Sweden. Today the Group has more than 300 members from 49 countries around the world. IRG provides a forum and networking system for wood preservation researchers, publishes more than 100 documents every year, arranges conferences and so forth. The web site does not offer any publications on line, however titles can be browsed and ordered through the site. Information offered is highly technical.

 

Web site: http://www.irg-wp.com/

 

Western Wood Preservers Institute

Vancouver, Washington, USA

Established in 1950 and representing the interest of the pressure treating wood products industry throughout western North America, WWPI provides a range of information on its web site for users of treated wood.

 

Web site: http://www.wwpinstitute.org/

 

INFORMATION ON TERMITES AND OTHER PESTS

 

Louisiana State University Agricultural Center

Web site: http://www.lsuagcenter.com/en/environment/insects/Termites/

 

University of Toronto, Urban Entomology Group

Web site offers information on biology and control of termites

Web site: http://www.utoronto.ca/forest/termite/termite.htm

 

University of Hawaii

Web site: www2.ctahr.hawaii.edu/oc/freepubs/index.asp

Click on “Household and Structural Pests” for fact sheets on termites and on termite barrier technologies.

 

University of Nebraska, Lincoln

Web site: http://www.ianr.unl.edu/ Search for “termite”.

 

Australian Pest Controllers Association

Web site: http://www.termite.com/

Very thorough information for builders and homeowners – useful outside Australia as well.

 

Further reading (termites):

Possibly the most comprehensive guidance on termite control is provided by two Australian standards:

• Australian Standard AS 3660.1-1995 Protection of buildings from subterranean termites. Part 1 New buildings. Standards Australia, Homebush,
• Australian Standard AS 3660-1993- Protection of buildings from subterranean termites. – Prevention, detection and treatment of Standards Australia, Homebush, NSW.
• You can purchase (but not view) these documents at this web site: http://www.standards.com.au/ – type “termite” in the search

 

INFORMATION ON MOLD

Mold, Housing and Wood

Written by an industrial hygienist and a wood mycologist in 2002. This 15-page paper is clear and well- referenced. Available on-line: http://www.wwpa.org/lumberandmold.htm 

The Condominium Home Owners Guide to Mold

A small booklet published by Canada Mortgage and Housing Corporation, providing useful and simple tips on prevention and cleanup. Available by phone – see CMHC listing at top of page.

Clean-up Procedures for Mold in Houses

Booklet published in 1993 by Canada Mortgage and Housing Corporation. Available from CMHC web site (Order Desk/Healthy Housing). See CMHC listing at top of page.

Mold in Housing: An Information Kit for First Nations Communities

Authored by Canada Mortgage and Housing Corporation, Health Canada and Indian and Northern Affairs Canada. Useful for anyone, not just First Nations/Native Americans. Addresses what to do about mold, in layperson’s language. Available only by phone from CMHC – see listing at top of page.

Fungal Contamination in Public Buildings: A Guide to Recognition and Management

Published by Health Canada in 1995, this 88-page, thorough and scientific report provides a protocol for investigating buildings with suspected fungal problems affecting human health. Available on-line at http://www.hc-sc.gc.ca/ewh-semt/alt_formats/hecs-sesc/pdf/pubs/air/fungal-fongique/fungal-fongique_e.pdf

Guidelines on Assessment and Remediation of Fungi in Indoor Environments Published in 2000 by the New York City Department of Health, Bureau of Environmental and Occupational Disease Epidemiology. Covers health issues and provides a protocol for assessment and remediation. Similar in scientific approach to the Health Canada document, however much shorter. Available on-line at http://home2.nyc.gov/html/doh/html/epi/moldrpt1.shtml

Moulds: Isolation, Cultivation, Identification

An on-line book, 1997, by David Malloch, Department of Botany, University of Toronto:

http://www.botany.utoronto.ca/ResearchLabs/MallochLab/Malloch/Moulds/Moulds.html

 

Mold Resources

The United States Environmental Protection Agency web site has comprehensive on-line information and many links on molds, cleanup and health: http://www.epa.gov/iaq/molds/index.html

Report of the Microbial Growth Task Force

Published by the American Industrial Hygiene Association, 66 pages, 2001, covers procedures for remediation of molds in buildings. Available on-line at: http://www.aiha.org/content/accessinfo/consumer/factsaboutmold.htm

 

OTHER WEB SITES OF INTEREST

University of Waterloo, Building Engineering Group

Waterloo, Ontario, Canada

The Building Engineering Group (BEG) is a multi-disciplinary group which undertakes research, development and demonstration (R,D & D) for the building industry. BEG is a non-profit, non-proprietary organization operating within the Civil Engineering Department. Operates an outdoor test facility for assessment of building envelope performance, the BEG hut.

Web site: http://www.civil.uwaterloo.ca/beg

Centre for Building Studies, Concordia University

Montreal, Quebec, Canada

The Centre is a research group within the Department of Building, Civil & Environmental Engineering. Research areas include building envelope performance, indoor environment, wind effects and more. The Centre has many advanced facilities for research – click on “Laboratories” to learn more.

Web site: http://www.bcee.concordia.ca/index.php/Centre_for_Building_Studiesindex.htm

National Building Envelope Council

Ottawa, Ontario, Canada

A forum for Canadian design professionals to share information and jointly pursue excellence in design, construction and performance of envelopes. Arranges annual conferences. Local chapters offer regular meetings, lectures, newsletters and so forth – click “Regional BECs”

Web site: http://www.nbec.net/

 

BOOKS ON WOOD DESIGN

Best Practice Guide, Wood Frame Envelopes in the Coastal Climate of British Columbia. Canada Mortgage and Housing Corporation, 1998. 211p, detailed drawings and 3D PowerPoint files. This is a comprehensive design guide. Also includes some background information on wood decay. Available from CMHC – see listing at top of page.

Building Envelope Rehabilitation: Consultant Guide and Owner/Property Manager Guide. Canada Mortgage and Housing Corporation, 2001. A two-volume set providing help in assessment and remediation of moisture-related damage in occupied buildings. Available from CMHC – see listing at top of page.

Builder’s Guides. J. Lstiburek, 1997, Building Science Corporation, Westford, MA, 303p. http://www.buildingscience.com/, 978-589-5100. Comprehensive guides available in four versions (one each addressing four different climate zones that cover all of North America) with detailed drawings covering design principles, foundations, framing, HVAC, plumbing, electrical, drywall and painting. Building science translated for the builder.

CSA S478-95 Guideline on Durability in Buildings. 1995, 93p, Canadian Standards Association, Etobicoke, ON. Advice on incorporating requirements for durability into the design, operation and maintenance provisions for buildings and their components. Overview of deterioration agents and life expectancies for various components.

Design of Wood Structures for Permanence. Anon., 1988, 17p, American Forest and Paper Association, Washington, DC. A very brief overview of general recommendations on good construction practice.

Available through the web site of the American Wood Council (see above, under “wood design and durability”).

Evaluation, Maintenance and Upgrading of Wood Structures, A guide and Commentary. Subcommittee of American Society of Civil Engineers, 1982, 428p, American Society of Civil Engineers, New York, NY. A somewhat dated guide to the technical aspects of inspection, evaluation, reinforcement, repair and rehabilitation of timber structures. Application of lessons learned to design and maintenance of new structures.

Guide to the Inspection of Existing Homes for Wood-Inhabiting Fungi and Insects. M.P. Levy, 1979, 104p, US Department of Housing and Urban Development. A well illustrated booklet which would be invaluable to anyone inspecting buildings for decay. Not as comprehensive as the BRE publication (see “Recognizing Wood Rot ” below), but more relevant to North American conditions.

Introduction to Wood Building Technology. Canadian Wood Council, 1997, 430p, Ottawa, ON. A technical book on wood-frame construction, it covers materials and properties, thermal insulation, fire protection, construction details, sound control and inspection and repair. Available from CWC – see listing at top of page.

Moisture Control Handbook: New low-rise, Residential Construction. J. Lstiburek and J. Carmody, 1991, 247p, US Department of Commerce, National Technical Information Service, Springfield VA. A guide for heating, cooling and mixed climates. How walls get wet through vapour diffusion, air leakage, condensation, and water leakage. Moisture control strategies. Building science explained.

National Building Code of Canada. National Research Council, 1995, 571p, Ottawa, ON. Minimum requirements to satisfy consumer health, safety and accessibility requirements of buildings. Protection of property is not considered a root objective of the building code consequently references to durability requirements are extremely limited. Very little guidance on how to implement the requirements of the code in terms of moisture control, compared to for example, fire control.

Recognising Wood Rot and Insect Damage in Buildings. A.F. Bravery, R.W. Berry, J.K. Carey and D.E. Cooper, Building Research Establishment, Garston, UK, 1992, 120p. A comprehensive and well-illustrated guide to distinguishing different types of wood-destroying organisms. More relevant to the UK and European building practice and organisms than to North America. Available at http://web.archive.org/web/20051104025920/http://www.bre.co.uk/ (click “bookshop”).

Selection and Use of Preservative Treated Wood. D.L. Cassens, W.C. Feist, B.R. Johnson and R.C. DeGroot, 1995, Forest Products Society, Madison, WI. An excellent guidebook for those contemplating using treated wood in a project.

Wood as a Building Material. W. Wilcox, E. Botsai, and H. Kubler, John Wiley & Sons Inc., 1991. 215 p, index, bibliography, many illustrations. Clear and concise, intended to be readable by building designers. Covers the properties and structure of wood, the relationship between wood and water, wood products, thermal properties, fire performance, decay and its prevention, wood finishing, design guidelines and wood identification.

Wood Protection Guidelines: Protecting Wood From Decay Fungi and Termites. Anon., 1993, Wood Protection Council, National Institute of Building Sciences. A very useful guidebook on controlling the conditions that favour attack by wood-destroying organisms.

Wood Reference Handbook, A Guide to the Architectural Use of Wood in Building Construction. Canadian Wood Council, 2000, 562p, Ottawa, ON. A wealth of basic information on wood characteristics, wood product properties, connections, structural wood systems, building completion, wood finishes, and fire safety. Minimal information on preservative treatment compared to fire control. Available from the Canadian Wood Council – see listing at top of page.

Finishes for Exterior Wood. R. Williams, U.S. Forest Products Laboratory, 1996. Excellent booklet available at low cost, by phone only – see the FPL listing near top of page. Also see FPL web site for on-line fact sheets on finishes.

Environmental product declarations (EPDs)

Stakeholders within the building design and construction community are increasingly being asked to include information in their decision-making processes that take into consideration potential environmental impacts. These stakeholders and interested parties expect unbiased product information that is consistent with current best practices and based on objective scientific analysis. In the future, building product purchasing decisions will likely require the type of environmental information provided by environmental product declarations (EPDs). In addition, green building rating systems, including LEED®, Green Globes™ and BREEAM®, recognize the value of EPDs for the assessment of potential environmental impacts of building products.

EPDs are concise, standardized, and third-party verified reports that describe the environmental performance of a product or a service. EPDs are able to identify and quantify the potential environmental impacts of a product or service throughout the various stages of its life cycle (resource extraction or harvest, processing, manufacturing, transportation, use, and end-of-life). EPDs, also known as Type III environmental product declarations, provide quantified environmental data using predetermined parameters that are based on internationally standardized approaches. EPDs for building products can help architects, designers, specifiers, and other purchasers better understand a product’s potential environmental impacts and sustainability attributes.

An EPD is a disclosure by a company or industry to make public the environmental data related to one or more of its products. EPDs are intended to help purchasers better understand a product’s environmental attributes in order for specifiers to make more informed decisions selecting products. The function of EPDs are somewhat analogous to nutrition labels on food packaging; their purpose is to clearly communicate, to the user, environmental data about products in a standardized format.

EPDs are information carriers that are intended to be a simple and user-friendly mechanism to disclose potential environmental impact information about a product within the marketplace. EPDs do not rank products or compare products to baselines or benchmarks. An EPD does not indicate whether or not certain environmental performance criteria have been met and does not address social and economic impacts of construction products.

Data reported in an EPD is collected using life cycle assessment (LCA), an internationally standardized scientific methodology. LCAs involve compiling an inventory of relevant energy and material inputs and environmental releases, and evaluating their potential impacts. It is also possible for EPDs to convey additional environmental information about a product that is outside the scope of LCA.

EPDs are primarily intended for business-to-business communication, although they can also be used for business-to-consumer communication. EPDs are developed based on the results of a life cycle assessment (LCA) study and must be compliant with the relevant product category rules (PCR), which are developed by a registered program operator. The PCR establishes the specific rules, requirements and guidelines for conducting an LCA and developing an EPD for one or more product categories.

The North American wood products industry has developed several industry wide EPDs, applicable to all the wood product manufacturers located across North America. These industry wide EPDs have obtained third-party verification from the Underwriters Laboratories Environment (ULE), an independent certification body. North American wood product EPDs provide industry average data for the following environmental metrics:

• Global warming potential;
• Acidification potential;
• Eutrophication potential;
• Ozone depletion potential;
• Smog potential;
• Primary energy consumption;
• Material resources consumption; and
• Non-hazardous waste generation.

Industry wide EPDs for wood products are business-to-business EPDs, covering a cradle-to-gate scope; from raw material harvest until the finished product is ready to leave the manufacturing facility. Due to the multitude of uses for wood products, the potential environmental impacts related to the delivery of the product to the customer, the use of the product, and the eventual end-of-life processes are excluded from the analysis.  

For further information, refer to the following resources:

ISO 21930 Sustainability in buildings and civil engineering works – Core rules for environmental product declarations of construction products and services

ISO 14025 Environmental labels and declarations – Type III environmental declarations – Principles and procedures

ISO/TS 14027 Environmental labels and declarations – Development of product category rules

ISO 14040 Environmental management – Life cycle assessment – Principles and framework

ISO 14044 Environmental management – Life cycle assessment – Requirements and guidelines

American Wood Council

Canada Green Building Council

Green Globes

BREEAM®

Annual Review Rules and Form EPD

Some engineered wood panel products, such as plywood and laminated veneer lumber (LVL) are able to be treated after manufacture with preservative solutions, whereas thin strand based products (OSB, OSL) and small particulate and fibre-based panels (particleboard, MDF) are not. The preservatives must be added to the wood elements before they are bonded together, either as a spray on, mist or powder.

Products such as OSB are manufactured from small, thin strands of wood. Powdered preservatives can be mixed in with the strands and resins during the blending process just prior to mat forming and pressing. Zinc borate is commonly used in this application. By adding preservatives to the manufacturing process it’s possible to obtain uniform treatment throughout the thickness of the product.

In North America, plywood is normally protected against decay and termites by pressure treatment processes. However, in other parts of the world insecticides are often formulated with adhesives to protect plywood against termites. 

Wall Types for Water Control

Building envelope experts generally speak of three or four different approaches to design of a wall for moisture control. Face seal walls are designed to achieve water tightness and air tightness at the face of the cladding. An example would be stucco applied directly to sheathing or masonry without a moisture barrier membrane such as building paper. Joints in the cladding and interfaces with other wall components are sealed to provide continuity. The exterior face of the cladding is the primary – and only – drainage path. There is no moisture control redundancy, i.e., there is no back-up system. A face seal system must be constructed and maintained in perfect condition to effectively control rain water intrusion. In general, these walls are only recommended in low risk situations, such as wall areas under deep overhangs or in dry climates. Concealed barrier walls are designed with an acceptance that some water may pass beyond the surface of the cladding. These walls incorporate a drainage plane within the wall assembly, as a second line of defense against rain water.

The face of the cladding remains the primary drainage path, but secondary drainage is accomplished within the wall. This drainage plane consists of a membrane such as building paper, which carries water down and out of the wall assembly. An example is siding or stucco applied over building paper. Concealed barrier walls are appropriate in areas of low to moderate exposure to rain and wind. Rainscreen walls take water management one step further by incorporating a cavity between the back of the cladding and the building paper. This airspace ventilates the back of the cladding, helping it to dry out. The cavity also acts as a capillary break between cladding and building paper, thereby keeping most water from making contact with the building paper. An example of a rainscreen wall is stucco or siding applied to vertical strapping over the building paper. Rainscreen walls are appropriate in high rain and wind exposures. An advancement of the rainscreen technology is the pressure-equalized rainscreen. These walls use vents to equalize the pressure between the exterior and the cavity air, thereby removing one of the driving forces for water penetration (when it is pushed through cracks due to high pressure on the face of the wall and low pressure in the cavity). These walls are for very high risk exposures.

Importance of an Overhang

In a rainy climate, an overhang is one of the simplest and most effective ways to reduce the risk of water intrusion. An overhang is an umbrella for the wall, and the deeper the better. A survey of leaky buildings in British Columbia commissioned by Canada Mortgage and Housing Corporation in 1996 showed a strong inverse correlation between depth of overhang and percent of walls with problems. However, even a small overhang can help protect the wall, largely due to its effect on driving rain. One important benefit of overhangs and peaked roofs often not appreciated is the effect of these elements on wind pressure. Wind-driven rain is typically the largest source of moisture for walls. An overhang and/or sloped roof will help direct the wind up and over the building, which reduces the pressure on the wall and thereby reduces the force of the driving rain striking the wall. This means water is less likely to be pushed by wind through cracks in the wall.

Minimize the Holes

Most rainwater problems are due to water leaking into the wall through holes. If care isn’t taken to protect discontinuities in the envelope, water can leak around window framing and dryer vents, at intersections like balconies and parapets, and at building paper joints, for example. Good design detailing and careful construction is critical! So is maintenance of short-life sealants like caulk around window frames. BC Housing-Homeowner Protection Office has updated the “Best Practice Guide for Wood-Frame Envelopes in the Coastal Climate of British Columbia” originally developed by Canada Mortgage and Housing Corporation and published “Building Enclosure Design Guide for Wood-Frame Multi-Unit Residential Buildings” with extensive information on design and construction detailing.

Use our Effective R calculator to determine not only the thermal resistance of walls, but also a durability assessment of the wall based on representative climate conditions across Canada.

Related Publications

For on-line design and construction tips, try the following:The Build a Better Home program, operated by APA-The Engineered Wood Association, runs training courses, operates a demonstration houses, and offers publications. The web site offers construction information and provides links to all relevant APA publications.

Building Enclosure Design Guide: Wood-Frame Multi-Unit Residential Buildings.

 

Safe Handling

Using common sense and standard safety equipment (personal protection and wood-working machinery) applies when working with any building products. Gloves, dust masks and goggles are appropriate for use with all woodworking. Here are a few key points specific to treated wood:

• Pressure-treated wood is not a pesticide, and it is not a hazardous product. In most municipalities, you may dispose of treated wood by ordinary garbage collection. However, you should check with your local regulations.
• Never burn treated wood because toxic chemicals may be produced as part of the smoke and ashes.
• If preservatives or sawdust accumulate on clothes, launder before reuse. Wash your work clothes separately from other household clothing.
• Treated wood used for patios, decks and walkways should be free of surface preservative residues.
• Treated wood should not be used for compost heaps where free organic acids produced early in the composting process can remove the fixed chemicals. It is, however, safe to use for growing vegetables in raised soil beds. If, after reading this, you are still concerned, place a layer of plastic sheet between the soil and the treated wood wall.
• Treated wood should not be cleaned with harsh reducing agents since these can also remove the fixed chemicals.

Environmental Concerns

All wood preservatives used in the U.S. and Canada are registered and regularly re-examined for safety by the U.S. Environmental Protection Agency and Health Canada’s Pest Management and Regulatory Agency, respectively. 

Wood preservation is not an exact science, due to the biological – and therefore variable and unpredictable – nature of both wood and the organisms that destroy it. Wood scientists are trying to understand more about how wood decays to ensure that durability is achieved through smart design and construction choices where possible, so that as a society we can be selective in our use of preservatives.

Comparing treated wood to alternative products

A series of life cycle assessments has been completed comparing preservative treated wood to alternative products. In most cases, the treated wood products had lower environmental impacts.

 

 

 

 

 

 

Click for consumer safety information on handling treated wood (Canada).

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60Sometimes it happens – wood in service suffers from decay. How can you identify decayed wood and what are the recommended actions to take? First, be sure you actually have decay. The wood may only be harmlessly discoloured, for any number of reasons. See the publication in the side bar for help if your wood is stained but you’re not sure why.

If wood is badly decayed, this will be quite obvious. The wood will be softer than normal and perhaps even be breakable by hand. Decayed wood often has a colour change, either darker or lighter than normal, although this could be due to weathering or could just be a stain. The wood may display an unexpected cracking pattern, or may look stringy- this is a sign of fairly advanced decay. If fungal growth is visible on the surface, the wood has quite likely already suffered strength loss even if this isn’t visibly obvious. However, do not rely on visual cues alone.

Wood can appear stained and yet be sound, or can appear normal yet have already suffered significant strength loss due to decay. Some researchers or engineers use the pick test to determine if the wood is sound. They insert the point of a knife at a shallow angle to the surface and attempt to lever up a thin splinter. If the wood splinters with longer fragments, it is likely sound. If instead it breaks or crumbles in small pieces over the blade, it could be decayed. Decayed wood breaks somewhat like a carrot snapping in half, at one section, versus the splintering along the length of sound wood. See our Biodeterioration page to learn more about the science of decay.

If you are still unsure whether or not you have decayed wood, you are advised to seek help from a wood restoration specialist.

How urgent is a decay problem? By the time you notice decay, the wood typically has lost substantial strength already. In cases where the decayed wood is supporting load you are strongly advised to contact a structural engineer or other appropriate expert to more thoroughly assess the problem and proceed with a repair.

A small, localized and non-critical case of decay may be a do-it-yourself project under some conditions. All decayed wood should be removed. If you are unable to remove the entire affected piece, remove the decayed portion plus an additional portion of adjacent wood beyond the visible decay. A rule of thumb is to remove an additional two feet (60 cm) of adjacent wood from each side, although this will of course depend on the extent of the decay. The removal of adjacent wood is because the fungus may have extended deep into the wood beyond the area of decay and may be ready to cause more damage in adjacent sound wood.

Then apply a field treatment to the remaining adjacent wood, such as a borate solution in roll-on, rod or paste form, before replacing the removed pieces. Use treated or naturally durable wood to replace the removed pieces. If damaged wood must be left in place, a penetrating epoxy can sometimes be applied as a stabilizer. In those cases and for best results in all wood repair projects we recommend you consult with a wood restoration expert.

Indoors, it is extremely important that you find the source(s) of the moisture that allowed wood decay fungi to grow. If you had wood decay in a location that is supposed to be dry, then you have a leak or a condensation problem that needs fixing to prevent any future problems. Look for primary and secondary sources of moisture. A short term leak may have allowed decay to start, for example, and condensation may be sustaining the decay. If the location of the decayed wood was outdoors or in a wet location, you need to use treated or naturally durable wood.

If you have building moisture problems on a large scale, you need to hire some experts and be prepared for a potentially substantial remediation project. Seek out a qualified consultant, who will begin by using a variety of techniques and tools to determine the extent of the damage. This will include a visual examination for staining, bulging, cracking, presence of water, and warping. Subsurface moisture penetration will be tested with probes and/or thermography.

In a building with wood structural members, the consultant will probably use a moisture meter to sample wetness of structural wood components in several locations. Based on the results of this investigation, the consultant will recommend a course of action for repair and future prevention. Canada Mortgage and Housing Corporation has developed a guide for building envelope rehabilitation, in two volumes: one for owners, one for consultants.

More Information

Click Here for a fact sheet Discolourations on wood products: Causes and Implications for help if your wood is stained and you’re not sure why.

Click here for more information on biodeterioration and the science of decay.

Click here for more information on remedial treatments.

Click here for links on decay assessment and other durability topics

Wood structures, properly designed and properly treated, will last indefinitely. This section includes guidance on specific applications of structures that have constant exposure to the elements.

Mass timber exteriors

Modern Mass Timber Construction includes building systems otherwise known as post-and-beam, or heavy-timber, and cross laminated timber (CLT). Typical components include solid sawn timbers, glue-laminated timbers (glulam), parallel strand lumber (PSL) laminated veneer lumber (LVL) laminated strand (LSL), and CLT. Heavy-timber post and beam with infill walls of various materials is one of the oldest construction systems known to man. Historic examples still standing range from Europe through Asia to the long-houses of the Pacific Coastal first nations (Figure 1). Ancient temples in Japan and China dating back thousands of years are basically heavy timber construction with some components semi-exposed to the weather (Figure 2). Heavy-timber-frame warehouses with masonry walls dating back 100 years or more are still serviceable and sought-after as residences or office buildings in cities like Toronto, Montreal and Vancouver (Koo 2013). Besides their historic value, these old warehouses offer visually impressive wood structures, open plan floors and resultant flexibility of use and repurposing. Building on this legacy, modern mass timber construction is becoming increasingly popular in parts of Canada and the USA for non-residential construction, recreational properties and even multi-unit residential buildings. Owners and architects typically see a need to express these structural materials, particularly glulam, on the exterior of the building where they are at semi-exposed to the elements (Figure 3). In addition wood components are being increasingly used to soften the exterior look of non-wood buildings and make them more appealing (Figure 4). They are anticipated to remain structurally sound and visually appealing for the service life. However, putting wood outside creates a risk of deterioration that needs to be managed. Similar to wood used for landscaping, the major challenges to wood in these situations are decay, weathering and black-stain fungi. This document provides assistance to architects and specifiers in making the right decisions to maximize the durability and minimize maintenance requirements for glulam and other mass timber on the outside of residential and non-residential buildings. It focusses on general principles, rather than providing detailed recommendations. This is primarily focussed on a Canadian and secondarily on a North American audience.

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Disaster Relief Housing

Shelter needs after natural disasters come in three phases:

1. Immediate shelter: normally supplied by tarpaulins or light tents
2. Transition shelter: may be heavy-duty tents or more robust medium-term shelters.
3. Permanent buildings: Ultimately permanent shelters need to be constructed when the local economy recovers.

Immediate and transition shelters are typically supplied by aid agencies. Light wood frame is ideal for rapid provision of medium- to long-term shelter after natural disasters. However, there are challenges in certain climates for wood frame construction that must be addressed in order to sustainably and responsibly build them. For example, many of the regions which experience hurricanes, earthquakes and tsunamis also have severe decay and termite hazards including aggressive Coptotermes species and drywood termites. In extreme northern climates, high occupancy loads are common and when combined with the need for substantial thermal insulation to ensure comfortable indoor temperatures, can result in condensation and mould growth if wall and roof systems are not carefully designed.

The desire of aid organizations to maximize the number of shelters delivered tends to drive down the allowable cost dictating simplified designs with fewer moisture management features. It may also be difficult to control the quality of construction in some regions. Once built, “temporary” structures are commonly used for much longer than their design life. Occupier improvements over the longer term can potentially increase moisture and termite problems. All of these factors mean that the wood used needs to be durable.

One method of achieving more durable wood products is by treating the wood to prevent decay and insect/termite attack. However, commonly available preservative treated wood in Canada may not be suitable for use in other countries. Selection of the preservative and treatment process must take into account the regulations in both the exporting and receiving countries, including consideration of the potential for human contact with the preserved wood, where the product will be within the building design, the treatability of wood species, and the local decay and termite hazard. Simple design features, such as ensuring wood does not come into contact with the ground and is protected from rain, can reduce moisture and termite problems.

Building with concrete and steel does not eliminate termite problems. Termites will happily forage in a concrete or masonry block buildings looking for wood components, furniture, cupboards, and other cellulosic materials, such as the paper on drywall, cardboard boxes, books etc. Mud tubes running 10ft over concrete foundations to reach cellulosic building materials have been documented. Indeed, termites have caused major economic damage to cellulosic building materials even in concrete and steel high-rises in Florida and in southern China.

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Timber bridges

Timber bridges are an excellent way to showcase the strength and durability of wood structures, even under harsh conditions, when material selection, design, construction and maintenance are done well. They could also be critical infrastructure elements that span fast rivers or deep gorges. Consequences of failure of these structures can be severe in loss of life and loss of access to communities. Durability is as critical as engineering to ensure safe use of timber bridges for the design life, typically 75 years in North America.

There are numerous examples of old wood bridges still in service in North America (Figure 1). The oldest are traditional covered bridges (Figure 2), three of which are around 190 years old. In Southeast China, Fujian and Zhejiang provinces have numerous covered bridges that are almost 1000 years old (Figure 3). The fact that these bridges are still standing is a testament to the craftsmen that selected the materials, designed the structures, built them, monitored their condition and kept them maintained and repaired. They would have selected the most durable wood species available, likely Chestnut or cedars in North America, china fir (china cedar) in southeast China. They would have adzed off the thin perishable sapwood exposing only the naturally durable heartwood. The fact the covered bridges around today all look similar is because those were the tried and tested designs that worked. They clearly designed those bridges to shed water with a wood shingle roof, vertical siding projecting below the deck and structural elements sheltered from all but the worst wind-driven rain. Any rain that did not drip off the bottom of the vertical siding and wicked up the end grain would also dry out reasonably rapidly. Slow decay that did occur at the bottom of these boards was inconsequential because it was remote from connections to structural elements. Construction must have been meticulously performed by experienced craftsmen. Those craftsmen may well have been locals that would continue to monitor the bridge over its life and make any repairs necessary. Of course, not every component in those ancient bridges is original, particularly shingle roofs that typically last 20-30 years depending on climate. These bridges have all been repaired due to decay and in some cases dismantled and re-built over the years for various reasons (e.g., due to changes in traffic loads, arson, flooding, fire, hurricanes, etc.). The Wan’an Bridge in Fujian is known to have been built in 1090, refaced in 1708 and rebuilt in 1845, 1932 and 1953. The apparently increasing frequency of rebuilding may suggest a loss of knowledge and skills, but all repairs and reconstruction prior to 1845 may not have been recorded.

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Permanent Wood Foundations

A permanent wood foundation (PWF) is a strong, durable and proven construction method that has a number of unique advantages over other foundation systems for both the builder and the homeowner. The first Canadian examples were built as early as 1950 and are still being used today. PWFs can also be designed for projects such as crawl spaces, room additions and knee-wall foundations for garages and mobile homes. Concrete slab-on-grade, wood sleeper floors and suspended wood floors can all be used with PWFs.

A permanent wood foundation is an in-ground engineered construction system designed to turn a home’s foundation into useable living space. A below-grade stud wall constructed of preservative treated plywood and lumber supports the structure and encloses the living space. PWFs are suitable for all types of light-frame construction covered under Part 9 (Housing and Small Buildings) of the National Building Code of Canada, under clauses 9.15.2.4.(1) and 9.16.5.1.(1). This includes single-family detached houses, townhouses, low-rise apartments, and institutional and commercial buildings. In addition, the recently revised CSA S406 standard, Specification of permanent wood foundations for housing and small buildings, allows for three-storey construction supported by PWF.

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Choosing a coating depends on what appearance is desired and what level of maintenance would be tolerable.  For many people, the basic choice is paint versus stain. The trade-off is often between maintenance frequency and appearance.

For many people, additional criteria include VOC emissions, ease of clean up, and cost.  See our Links page for web sites and books with detailed information on choosing and applying wood finishes.  Read our About exterior wood coatings page for an understanding of the differences between paints and stains, pigmented versus clear coatings, and so forth.

Because exterior wood shrinks and swells with moisture changes, the coating needs to be flexible. Flexibility varies by product – some products may be clearly identified as suitably flexible for wood’s dimensional changes.  Water-borne coatings are generally more flexible than alkyds. Coatings containing urethanes tend to be more flexible than coatings containing acrylics.

For factory finishing with transparent coatings, with special considerations for UV and mildew control, please see our fact sheet Factory Finishing with Transparent Coatings: Requirements for Maximizing Longevity.

Special Considerations

If a coating is desired for a wear surface such as a deck or stairs, consult carefully with the coating manufacturer to choose the right product for this demanding application.  All coatings will be challenged by foot traffic and increased exposure to weather in a horizontal application.  High traffic routes will show wear faster than other areas. Paints and other thick film-formers may fail quickly in this situation, and a time-consuming refinishing process will be necessary each time the coating fails.  Hence many people will find a stain the more convenient choice for decks and stairs.

Knots may require a bit of extra care as some wood extractives or resin may leach out or bleed. Extractive bleeding can cause discolouration, but this can usually be prevented by applying special stain-blocking primers. In some species, especially the pines and Douglas-fir, knots and pitch pockets contain resin. The resin can bleed and may discolour the finish, leave hard beads of resin on the surface, or may otherwise interfere with the coating bond. The best way to prevent this is to purchase kiln-dried wood where the resin should be set (hardened and fixed in place). If painting is desired, choose higher grades of lumber as these will have fewer knots, and choose kiln-dried lumber if using a resinous species.

If siding or sidewall shingles are to be painted, the US Forest Products Laboratory (USFPL) recommends they be backprimed.  This application of a coating to the back side will plug the wood pores, preventing extractive bleed without blocking water vapour transmission and also preventing liquid water uptake.

If possible, round out any sharp corners for best coating adhesion on these edges – for example, a square-edged stair tread will show coating degradation quickly, but bullnosed stair tread edges will retain a coating much longer.  This is because a coating applied to a corner tends to pull away from the corner, leaving a much thinner layer there than elsewhere.

Surface Preparation

Durability of any finish is highly dependent on proper application, which includes good preparation of the surface to be coated.  Specific details on surface preparation depend on what condition the wood is to begin with – read on for tips that apply to various scenarios.

Surface Preparation for Fresh Wood

While fresh, clean wood can be coated without surface preparation, a light sanding with 100 grit sandpaper (and dust removal) can double the service life of some water-based coatings. For best results apply a coating to a fresh wood surface as soon as possible after planing or sanding.  If exposed to rain and sun for more than two weeks, adhesion of coatings will not be as good. The surface must also be free of anything that will interfere with coating adhesion, such as dirt, damaged wood fibres and moisture. Grade stamps on wood should also be removed before applying a semitransparent stain, preferably by sanding.

Cleaning

If there are discolourations caused by dirt, iron stains or other discolourations on the wood surface, cleaning may be desired. It is always preferable to achieve cleaning with sanding when possible.  Another safe way to clean wood without damaging the surface is to simply use a garden hose, with or without a pressure nozzle.  Use pressure-washing only with extreme care as it can damage wood, especially low-density species such as western red cedar.  The pressure should be kept at a minimum, and never hold the nozzle in one place for a long time.  If necessary, use a little bit of dish detergent, and lightly scrub (not with steel wool, as this will leave iron stains) in the direction of the grain for any stubborn discolourations.  For discolourations that resist soap-and-water cleaning, chemical cleaners will be effective.  The chemicals in commercial wood cleaners can be caustic soda (sodium hydroxide), sodium metasilicate, oxalic acid, citric acid, phosphoric acid, borax or some mixture. Wood cleaners containing caustic soda at a 1% –  2% solution will remove nearly all discolourations with the least damage to wood. Some acid cleaners are especially effective for removing extractive stains and iron stain.  Bleach is commonly used for cleaning wood, but we do not recommend this, since a poor wood substrate will usually be left behind for subsequent coating.  Resin (pine pitch) can be generally removed with mineral spirits. Please note that all acidic or alkaline chemicals need to be thoroughly rinsed off before coating. Chemicals can be toxic, corrosive and harmful, so handle all these chemicals with care and follow all manufacturer’s instructions.

Surface Preparation for Aged Wood

Wood coatings need a fresh surface or the coating simply won’t last. The longer wood has been allowed to weather, the poorer the coating adhesion. If a fresh surface is allowed to weather or age outdoors for more than two weeks, coating adhesion will deteriorate. This is mainly due to wood damage from sunlight. Weathered wood surfaces usually have a higher acidity, higher contact angle, and lower surface energy.

Restoring an aged wood surface is necessary before applying a coating.  The damaged (aged/weathered) wood fibres must be removed, exposing fresh wood.  Also, any discolourations will typically be removed along with the damaged fibres, so the process of restoration is simultaneously a cleaning process.  Wood restoration can be achieved with sanding or with chemicals, but sanding is always preferable when possible.  Sanding can be done by hand or machine until the true wood colour shows. Then brush off the sawdust and apply the coating immediately.  For many jobs, a chemical method will be far easier.  Read the label of each product to identify the active components.  In general, caustic soda (sodium hydroxide) is the best chemical choice for both cleaning and restoration.  It effectively removes weathered wood fibres from the surface and leaves the surface at a suitable pH for coating.  Oxalic acid is also commonly identified as a wood restorer, however, it is only effective at discolouration removal and does not remove the damaged wood fibres from the surface – in other words, it is not restoring the wood to be an appropriate substrate for a coating.  However, oxalic acid can be used to return the original wood colour after the use of sodium hydroxide.  Sodium hydroxide will slightly darken the wood, and, if this is undesirable, simply rinse the wood with oxalic acid after restoration with sodium hydroxide.  Please note that all these chemicals must be handled with care and all manufacturer’s instructions should be followed, as the chemicals can be toxic, corrosive and harmful. Where the wood is close to plants, wet down the leaves with a garden hose prior to and after chemical use. Wood surfaces should also be thoroughly rinsed with water before coating.

Maintenance

Maintaining a coating means giving it a wash occasionally, watching for signs that the coating is losing integrity, and applying a fresh coat before full failure sets in.  If a coating is reapplied before the last coat has failed, the stripping process may not be necessary. It’s time to apply another coat when paint has worn down to the primer, or if the coating colour has undesirably faded, or if the surface of water-repellent treated wood no longer beads water.  Then wash or brush off dirt and apply a new coat.  Any areas showing failure (the coating has lifted from the surface or cracked, or bare wood is showing) can be spot-treated.  Remove any loose pieces of paint and use sandpaper to feather the edges of adjacent sound paint so the transition won’t be evident through the new paint layer.  Also sand away any weathered wood.  For large scale failure, refinishing will be necessary. For all coating systems, there is a limit to the number of coats a surface can support. When the coating gets too thick, refinishing will also become necessary.

Refinishing

Refinishing a coating means stripping off the old coating and starting over.  This is necessary when large areas of the coating have failed, or the coating is getting too thick for refinishing, or if a decision is made to change the type of coating.  A coating has failed when it no longer adheres to the wood surface.  If the coating has bubbled, cracked, or peeled, it must be removed.  If the coating has simply faded but otherwise appears to still be well-bonded, it may not need to be removed.  When a change of coating type is desired, the new coating may be incompatible with the old coating – to ensure a good bond for the new coating, strip off the old one.  Remove coatings by sanding or with a chemical product.  Sanding has advantages over chemical stripping in restoring the fresh wood surface, but even if sanding is done by machine, it is still very labour-intensive for large painted areas typical of outdoor projects.  Sandblasting is not recommended except for large timbers and logs, as it will pit the wood and is hard to keep away from elements like window frames.  Powerwashing will only remove loose paint, leaving behind paint that is still adhered.  So, a chemical approach is generally regarded as the most effective and least labour-intensive way to strip a coating.  Sodium hydroxide at a 6% –  8% dilution is the recommended chemical for stripping – and offers the additional benefits of cleaning discolourations and restoring the wood surface at the same time.  Products containing sodium hydroxide are corrosive and should be prevented from touching skin. Follow manufacturers’ instructions.  There are also other chemical products for stripping coatings in the market.  After stripping with chemicals, always give the wood a final rinse with water.  Many projects will still require some light sanding around stubborn stains or heavily damaged wood.

• Select heartwood where possible to minimize nutrient content of wood surfaces and prevent nutrients migrating through the coating to support fungal growth on the surface.

• Round all corners to minimum 5 mm radius to eliminate sharp edges where coating can thin out.

• Prepare surface by sanding with 100 grit sandpaper to physically and chemically activate the surface.  Pretreatment and coating should be applied immediately after sanding. Research shows sanding can double coating life.

• Pretreat with an aqueous formulation containing a UV absorber designed to absorb the visible light that must penetrate transparent coatings to permit the wood to be visible. If the subsequent coating is not completely opaque to UV light, a hindered amine light stabilizer should be added to the visible light protection system. Not only does a visible light protection system prevent degradation of the wood-coating interface, it also prevents release of lignin breakdown products that can be used as a food source by black-stain fungi and prevents light induced breakdown of the biocide components. This pre-treatment must also contain three low-dose carbon-based biocides with differing chemistries to provide cross protection against detoxification and with complementary spectra of activity providing resistance to the full range of black-stain fungi. It should ideally have water repellent properties and must maintain wood surface pH close to neutral or slightly alkaline.

• Apply a transparent water-based catalyzed urethane coating, containing organic and inorganic UV absorbers with absorbance that extends from UVB through to the high-energy part of the visible spectrum (violet light). The coating must virtually eliminate UV from penetrating to the wood, preventing breakdown of wood, biocides and water repellents. This coating will be formulated to be damp-wood friendly to allow application soon after pre-treatment. It will contain no nutrients for fungal growth. It must have an optimum combination of moisture excluding efficiency and vapour permeability to minimize moisture uptake and allow drying after rain. The first coat to be designed to penetrate and bond to the wood, subsequent coats to be designed to ensure maximum intercoat adhesion without sanding between coats. Sufficient coats to be applied to give a film thickness no less than 60 microns to minimize the ability of black-stain fungi to penetrate the film with their infection pegs. The surface layer to have sheeting rather than beading properties to ensure rapid drying after rain or dew, reducing the time available for spore germination.

Additional detailed information on coating wood surfaces has been assembled by the Joint Coatings and Forest Products Committee (http://www.fpl.fs.fed.us/documnts/pdf2004/fpl_2004_bonura001.pdf, 2004).

How long will an exterior wood coating last?  Anywhere from a few months to 20 years or more, depending on the choice of product, how it was applied, and how severe the environment.

Paints tend to last the longest, assuming they are applied properly (see Choosing and applying exterior wood coatings page).  But the range of lifespan for a paint coating is very large.  A low quality product badly applied to a weathered wood surface may barely last two years.  If everything is done right, the coating might last 20 years.  High quality paints and stains generally last longest, and coatings that are in locations protected from sunlight and water tend to last longer.

Stains and water repellents have much shorter lives than paints, but are easier to maintain.  This is one of the reasons they are a popular choice for stairs and decks.  Depending on the degree of exposure to sun, water, foot traffic, and the pigment amount in the stain, expect a life of 1 to 2 years for a stain applied to deck boards and 2 to 5 for a stain applied to products that are not subject to wear.  Water repellents generally last 6 to 12 months.

Results from numerous tests on exterior wood finishes by many experts in this field, particularly by the US Forest Products Lab (USFPL), are summarized below.  See the USFPL link for more information.

Effect of wood anatomy

• Coatings, particularly solid colour stains and paints tend to last longer on dimensionally stable species such as western red cedar, eastern white cedar and Alaska yellow cedar, as these will shrink and swell less than other species and will therefore put less stress on the coating bond.  However deck stains will not last as long on low density species such as western red cedar due to wear.
• Coatings last longer on wood with narrow latewood bands (the dark part of the annual ring) due to density differences between the earlywood (the light part of the ring) and the denser latewood.  The southern pines are characterized by their wide bands of latewood, and therefore these species are considered to be somewhat poor for painting.
• The amount of extractives or resin in wood also affects coating performance. Special primers can be used to block water-soluble extractives, and kiln drying is most effective for fixing resin in wood.  Nutrients in wood can migrate through the coating to support fungal growth on the surface, and heartwood can be chosen to minimize the nutrient content in wood.

Effect of grain

• Finishes last longer on vertical (also called edge grain) versus flat grain, as these surfaces will shrink and swell less and therefore put less stress on the coating bond.  However, it can be difficult to specify type of grain when ordering a product.  Western red cedar and redwood may be available in a premium grade, which will likely be all heartwood, vertical grain.
• If using flat grain, place it bark side out or up if possible, because the grain is less likely to raise on that side, particularly in species with dense latewood bands such as the southern pines, and raised grain is a problem for coating adhesion. This is not an issue when using vertical grain products. Placing bark side out also minimizes checking.

Effect of surface roughness

• Rough-sawn (saw-textured) or roughened wood creates a better coating bond and thicker coating buildup than smooth wood.  The life of a coating can be substantially extended if the wood is roughened.

Effect of sanding

• Sanding (100 grit) can double the life of a coating, for both weathered and freshly planed wood.  This is because sanding removes any damaged surface fibres and also changes the surface chemistry to improve bonding of the coating.

Effect of wood preservatives

• Semitransparent stains last longer when applied to CCA-treated wood – treated wood purchased prior to 2004 was probably treated with CCA.  Research is under way on finishing for wood treated with new preservatives. Protection measures regarding use of treated wood apply when coating preservative-treated wood.

Effect of bluestain

• Bluestain is caused by fungi, and bluestained wood is more permeable than unstained wood, therefore it may absorb more coating.  Make sure to apply sufficient coating.

Effect of weathering

• Sunlight quickly degrades the ability of a wood surface to bond with a coating.  Research has shown a tremendous difference in paint performance on weathered versus unweathered wood.  Paint on boards with no exposure to weather prior to painting lasted at least 20 years.  Boards that had weathered for 16 weeks prior to painting began showing cracks in just 3 years.  For maximum coating life, sand the surface if the wood has been exposed to any sunlight at all, particularly if for more than two weeks.

Effect of product manufacturing

• Plywood:  Coatings on plywood are challenged by the small cracks (face checks) on the surface that are caused by the lathe when the veneer is cut from the log during manufacturing.  As the plywood goes through moisture cycling outdoors, these cracks tend to get larger and stress the coating bond.  Plywood surface, edges and joints in outdoor applications should be protected, and coatings and other products for helping plywood resist cracking can be applied to prevent moisture ingress.  Generally a good stain can effectively protect plywood. Since checking in stained plywood usually occurs during the first six months of outdoor exposure, best coating results can be obtained by applying a first coat and allowing any checking to occur, then six months or so later applying a second coat.  Paints can fail quickly on plywood, unless efforts are made to reduce moisture uptake and also to use flexible products to accommodate dimensional changes of the wood. Roughening the surface is also important. For plywood protection and other issues with plywood, see the recommendations from the Canadian Plywood Association (http://www.canply.org/pdf/main/plywood_handbookcanada.pdf).
• Finger-jointed products: Coatings may perform differently on different parts of these products, as they are not likely to be uniform in grain orientation, in heartwood versus sapwood content, or even in species.  Roughen the surface to extend the life of the coating and minimize these differences. Apply primer and paint all sides if possible to minimize moisture absorption.

Effect of priming

• Field tests have shown that coatings last much longer when a primer coat is used.
• Field tests have shown that siding or shingles last much longer if they are back-primed.

Effect of design and installation

• Use good design and installation practices to protect wood from sunlight and water, and prevent moisture accumulation in wood structures.
• By providing adequate clearance to grade, adequate roof overhang, rainscreen wall and back-priming, the coating life on siding can be effectively extended.
• If using flat grain, place the bark side out if possible to avoid raised grain.
• Use corrosion-resistant fasteners.